The present invention generally relates to a reproduction method and a reproduction apparatus for video signals as well as audio signals, and more particularly to the method and apparatus that skip some of the displayed pictures when the data coded through the MPEG method are reproduced.
According to the promulgated standard of MPEG coding method, three coding methods are available for coding video signals, i.e. intra frame coding method, predictive coding method and bi-directionally predictive coding method. The pictures coded by the respective methods are referred to as I-picture, P-picture and B-picture. When a picture including I-picture is collected to form a group called a group of pictures (GOP), motion picture information that takes a large capacity can be coded efficiently.
In a conventional MPEG reproducing method, coded data have been decoded and processed in a unit of a GOP or a unit of access that comprises not less than one GOP. For instance, when a frame Y is reproduced following after a distant frame X, a skip from X to Y is practiced. The skip is controlled in a unit of a GOP, more specifically, in a unit of access to data stored in a medium. The skip from the frame X to frame Y thus cannot expect a high accuracy. This is described hereinafter with reference to FIG. 5.
First, a conventional method is described that illustrates the conventional reproduction method. FIG. 5(a) shows a skip reproducing operation of the data coded with the MPEG method on tracks of a disc storage medium such as an optical disc. This is an example of the case where data coded with the MEPG method are stored and placed in a storage medium.
FIG. 5(b) shows data placement on a track of the disc. In this case, one GOP is handled as an access unit to the data on the track. Assume that a frame X and a distant frame Y is sequentially reproduced by skipping, where the frame X belongs to XGOP, a first GOP, and the frame Y belongs to a distant YGOP, a second GOP.
FIG. 5(c) illustrates a structure of a GOP in a unit of a frame, where nine frames constitute one GOP. The GOP shown in FIG. 5(c) comprises the following frames:
one intra coded frame (I-picture);
two predictive coded frames (P-pictures); and
six bi-directionally predictive coded frames (B-pictures).
Each frame is indicated with one block in FIG. 5(b), and respective frames are identified with the letters xe2x80x9cIxe2x80x9d, xe2x80x9cPxe2x80x9d and xe2x80x9cBxe2x80x9d above the respective blocks. Each frame of respective GOPs is numbered 1-9, and 11-19.
Assume that the frame X is the fifth frame (B-picture) of XGOP, and the frame Y is the sixth frame (B-picture) of YGOP.
FIGS. 5(d), 5(e) and 5(f) show the frames in which data are stored, the frames to be decoded, and the frames to be displayed when a skip is practiced from XGOP including the frame X to YGOP including the frame Y, and these two frames are reproduced sequentially. In these figures, each frame is identified and numbered in the same manner as in FIG. 5(c).
As shown in FIG. 5(a), alter XGOP is reproduced, some GOPs are skipped before the data immediately before the YGOP""s starting point are reproduced. In this operation, as shown in FIG. 5(d), the data of a frame 11 (I-picture) is placed immediately after the frame 9.
Regarding a decoding operation, as shown in FIG. 5(e), every frame where data are stored, as shown in FIG. 5(d), is decoded. Regarding a display operation, as shown in FIG. 5(e) and FIG. 5(f), every decoded frame is displayed.
As described above, YGOP is reproduced following XGOP, i.e. pictures are reproduced in a unit of a GOP, more specifically, in an access unit to the data stored in the storage medium.
In this conventional way, however, the frames 6-15 are often displayed as shown in FIG. 5(f) although they are displayed unnecessarily. A frame and its distant frame are thus not reproduced in sequence as requested. This is a problem of the conventional art.
The present invention provides a method and an apparatus with which unnecessary displayed frames can be reduced so that an accuracy of skip reproduction can be improved.
When the frames between X in XGOP and Y in YGOP are skipped, the data are decoded and displayed in the following manner.
(a) At the time when the frame X is decoded and displayed, stop the decoding process of XGOP, and freeze the display of frame X, then skip toward the frame Y in YGOP. Store the front data (i.e., the data situated at the beginning of the frame) and onward in YGOP into a storage medium, and decode as well as display the front frame and onward in YGOP.
(b) Keep on displaying the frame X, while storing the front data and onward in YGOP. Decode but not display the front frame of the I-picture in YGOP to the frame Y therein. At the time when the frame Y is decoded, start displaying the decoded frames.
(c) Keep from being decoded the bi-directionally coded frames between the first frame of YGOP and the frame Y, but decode all the frames including and after the frame Y.
This method improves the accuracy of skip reproducing, and reduces the freezing time of displaying a picture from which the skip starts.